Many techniques for stereo recording have been proposed. The original stereo recording technique, proposed by Blumlein in the 1930's, uses two dipole (figure of eight) microphones pointing towards directions +−45 degrees relative to the forward direction. Blumlein proposed the use of “coincident” microphones, that is, ideally the two microphones are placed at the same point. In practice, coincident microphone techniques place the microphones as closely together as practically possible, i.e. within a few centimeters.
Alternatively, one can use a coincident pair of microphones with other directionality for stereo recording, such as two cardioid microphones. Two cardioids have the advantage that sound arriving from the rear is attenuated (such as undesired noise from an audience).
Coincident microphone techniques translate direction of arrival of sound into a level difference between the left and right microphone signal. Thus, when played back over a stereo sound system, a listener will perceive a phantom source at a position related to the original direction of arrival of sound at the microphones.
Due to the limited directionality of most microphones, the responses often overlap more than desired, resulting in a recorded stereo signal with the left and right channel more correlated than desired. Diffuse sound results in left and right microphone signals which are more correlated than desired, having the effect of a lack of ambience in the stereo signal.
For multi-channel surround recording, this. problem of more than desired overlap of the responses is much more severe due to the necessity of using more microphones (with the same wide responses). There is not only a lack of ambience in the recorded surround signal, but also localization is poor, due to the high degree of cross-talk between the signals.
To circumvent the problem of too highly correlated signals, stereo and surround signals are often recorded using spaced microphones. That is, the microphones are not placed very close to each other, but at a certain distance. Commonly used distances between microphones are between 10 centimeters up to several meters. In this case, sound arriving from different directions is picked up with a delay. between the various microphones. If omnidirectional microphones are used, there is a delay and sound is picked at with a similar level by the various microphones. Often directional microphones are used, resulting in level differences and delays as a function of direction of arrival of sound. This technique is often denoted AB technique and can be viewed as a compromise between coincident and spaced microphone techniques.
For achieving a compromise-free stereo or surround recording, one would need coincident or nearly coincident microphones with a directionality higher than conventional first order microphones. The high directionality will improve perceived localization, ambience, and space when listening to the recording. In summary, one of the most important limitations of stereo and surround sound recording is, that highly directional microphones suitable for music recording are not available.
More directional second or higher order microphones have been proposed but are hardly used in professional music recording due to the fact. that they have lower signal to noise ratio and lower signal quality.
An alternative for getting a high directionality is the use of microphone arrays and the application of beamforming techniques. Beamforming has a number of limitations which have prevented its use in music recording. Beamforming is by its nature a narrow band technique and there is a dependency between frequency and beamwidth. In music recording, at least a frequency range between 20 Hz and 20000 Hz is used. It is very difficult to build a beamformer with a relatively frequency invariant beamshape over this large frequency range. Further, an array with many microphones would be needed for achieving good directionality over a wide frequency range.
While adaptive beamforming effectively improves directionality for a given number of microphones, it is not suitable for stereo or surround recording because it does have a time-variant beamshape, and thus is not suitable for translating direction of arrival of sound into level differences, as is needed for good localization.